In conventional cellular radio systems, geographical areas are divided up into a plurality of adjoining cells, in which mobile stations within a cell communicate with a base transceiver station. In general, each mobile (or set of mobiles sharing a multiplexed channel) communicating with a base station in a cell uses a different carrier frequency to other mobiles in the cell, to avoid interfering with the other mobiles. Thus the number of mobiles which can be served in a cell is limited by the number of available carrier frequencies. There is increased capacity demand for use of cellular radio systems, however the frequency band within which cellular radio systems operate is limited in width, and so to provide increased capacity in the system, available carrier frequencies are re-used from cell to cell.
The re-use of frequencies in a locality is restricted by co-frequency interference between different cells which re-use the same or close frequencies and which are geographically close to each other. To obtain maximum capacity in a system comprising a plurality of cell areas, cellular radio system designers aim to re-use as many different carrier frequencies of the set of available carrier frequencies as possible in each cell. However there are limits on the re-usage of carrier frequencies in a cell due to other potentially interfering signals, particularly from:
(1) interference between a carrier frequency in a first cell and an identical frequency re-used in neighboring cells and (2) interference between a carrier frequency used in a first cell and adjacent carrier frequencies used in neighboring cells.
The minimum physical distance between geographic cells which re-use a same carrier frequency or an adjacent carrier frequency is limited by the required quality of signals received at the carrier frequency. One metric used to describe the quality of the signal is referred to in the art as the carrier to interference ratio (C/I ratio). The C/I ratio is a ratio of signal strength of a received desired carrier frequency to a signal strength of received interfering carrier frequencies and noise. A number of physical factors can affect the C/I performance in cellular systems including reflections from buildings, geography, antenna radiation patterns, mobile station transmitting power, and mobile station locations within a cell. In general, calculating the distances between cells which re-use an interfering carrier frequency is a complex problem, however a general approach to the calculations may be found in Mobile Cellular Telecommunications Systems by William Chien-Yeh Lee published by McGraw Hill Book Company, New York 1989.
Taking as an example a Digital Amps TDMA (time division and multiple access) system having available 12.5 MHz of frequency spectrum, for example in the 850 MHz band, individual carrier frequencies are spaced apart from each other centered at spacings of every 30 KHz, giving a total of 416 carrier frequencies available across the network as a whole. The 416 carrier frequencies are partitioned so that individual carrier frequencies are re-used from cell to cell.
Taking as an example a base station re-use factor n of 7 (n=7), for center-excited cells each cell is allocated 416.div.7=59 carrier frequencies per cell.
However, with a base station re-use factor of n=4, this gives 416.div.4=104 carrier frequencies per cell, resulting in a higher capacity than for an n=7 re-use factor. At a base station re-use factor of n=4 cells which re-use a same carrier frequency (the frequency re-use cells) are closer to each other than at a base station re-use factor n=7, resulting in more interference, and a lower C/I ratio in the base station re-use factor n=4 case than in the base station re-use factor n=7 case. To implement the lower base station re-use factor (n=4) frequency, re-use cells must be closer together than with a higher base station re-use n=7. However, the distance between the re-use cells must be great enough so that the carrier to interference ratio is high enough to allow the cellular radio telecommunications apparatus to distinguish signals at each re-used carrier frequency in one cell from the interfering frequencies present in other cells across the network. The C/I performance is a limiting factor in implementation of a lower base station re-use factor.